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here is related with [non]premultiplication problems, but once you manage
to fix the save-to-png routine, you should be able to check that the brush
you have in step 5 is correct.

>
> I can wrap up my current C# work and put it online if that will work - I
> don't know if you can look at that at all, otherwise I'll put together a C
> application for it.

Your description has been quite detailed, so I hope we can help you fix
the problem without much additional work.

Andrea